feat(ocr): Add docTR OCR engine with metrics infrastructure

Add docTR as primary OCR engine with 2-tier sequential processing,
OCR metrics tracking, and simplified engine selection.

Features:
- docTR OCR engine with light+medium preprocessing tiers
- doctr_plus mode with early exit optimization (~65% fast path)
- OCR metrics dashboard with per-engine statistics
- User OCR preference persistence
- Parallel worker pool for OCR processing
- Cross-validation for extraction quality

Engine options: tesseract, doctr, doctr_plus (recommended), paddleocr

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/modules/data_entry/db/models/__init__.py

@@ -2,6 +2,7 @@
 from .receipt import Receipt, ReceiptAttachment, ReceiptStatus, ReceiptType, ReceiptDirection
 from .accounting_entry import AccountingEntry, EntryType
 from .nomenclature import SyncedSupplier, LocalSupplier, SyncedCashRegister
+from .ocr_settings import UserOCRPreference, OCRJobMetrics, OCRMetricsSummary, OCREngine
 
 __all__ = [
     "Receipt",
@@ -14,4 +15,9 @@ __all__ = [
     "SyncedSupplier",
     "LocalSupplier",
     "SyncedCashRegister",
+    # OCR Settings & Metrics
+    "UserOCRPreference",
+    "OCRJobMetrics",
+    "OCRMetricsSummary",
+    "OCREngine",
 ]